Significance:A newly discovered disease, referred to as the "thousand cankers disease," is killing black walnut (Juglans nigra) trees in Colorado and possibly other areas of the southern and western United States. The disease is likely caused by the fungus Geosmithia spp. (Ascomycetes: Hypocreales), and is spread by the walnut twig beetle Pityophthorus juglandis (Coleoptera: Curculionidae).

In its native range, this insect does not kill its usual hosts (e.g., Arizona walnut - Juglans major), although it can cause dieback on twigs and smaller branches of stressed trees. Native to Mexico, Arizona, and New Mexico, the beetle appears to be expanding its range north into other regions, including Colorado, where it has been found attacking a new host, black walnut (Juglans nigra). On this host the beetle is more aggressive, attacking larger diameter branches and causing major branch dieback. In addition, the insect has developed an association with Geosmithia spp., a fungus known to associate with bark beetles of hardwood trees. As the walnut twig beetles bore into the bark of black walnut, the fungus is introduced; this leads to the development of fungal cankers under the bark, restricting the flow of nutrients and eventually killing the tree, sometimes within one year.

Over the course of just a few years, the insect-fungus infestation has killed most of the black walnut trees in cities such as Boulder and Colorado Springs, Colorado. The disease has also been found in other parts of the State, as well as in parts of New Mexico. In addition, samples of J. nigra and J. regia (English or Persian Walnut), with both the vector and the fungus, have recently been collected in Utah.

Issues of Concern:
This new insect-fungus association appears to represent a significant threat to black walnuts. According to other reports, dieback and mortality of black walnut have also been reported in other areas of the United States (e.g., Utah, Missouri, the Willamette Valley in Oregon, and in Idaho), although further investigation is required to determine the exact cause(s) of tree mortality in those areas. The native range of black walnut in North America is very substantial and includes almost the entire eastern half of the United States up into southern Ontario, Canada. Should this insect-fungus disease complex ever be introduced into the native range of black walnut, the damage could be substantial. Until more is known about this fungus-insect associated disease, caution should be exercised when moving any materials (e.g. firewood) that could spread the fungus and/or vector into areas where walnuts are grown.

Warning: The information in this alert has not been confirmed with the appropriate National Plant Protection Organization and is provided solely as an early warning. Please use the above information with caution.